parse-options: add one-shot mode

This is to help reimplement diff_opt_parse() using parse_options().
The behavior of parse_options() is changed to be the same as the
other:

- no argv0 in argv[], everything can be processed
- argv[] must not be updated, it's the caller's job to do that
- return the number of arguments processed
- leave all unknown options / non-options alone (this one can already
  be achieved with PARSE_OPT_KEEP_UNKNOWN and
  PARSE_OPT_STOP_AT_NON_OPTION)

This mode is NOT supposed to stay here for long. It's to help
converting diff/rev option parsing. Once that work is over and we can
just use parse_options() throughout the code base, this will be
deleted.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -27,7 +27,8 @@ enum parse_opt_flags {
 	PARSE_OPT_STOP_AT_NON_OPTION = 2,
 	PARSE_OPT_KEEP_ARGV0 = 4,
 	PARSE_OPT_KEEP_UNKNOWN = 8,
-	PARSE_OPT_NO_INTERNAL_HELP = 16
+	PARSE_OPT_NO_INTERNAL_HELP = 16,
+	PARSE_OPT_ONE_SHOT = 32
 };
 
 enum parse_opt_option_flags {
@@ -169,10 +170,18 @@ struct option {
 	  N_("no-op (backward compatibility)"),		\
 	  PARSE_OPT_HIDDEN | PARSE_OPT_NOARG, parse_opt_noop_cb }
 
-/* parse_options() will filter out the processed options and leave the
- * non-option arguments in argv[]. usagestr strings should be marked
- * for translation with N_().
+/*
+ * parse_options() will filter out the processed options and leave the
+ * non-option arguments in argv[]. argv0 is assumed program name and
+ * skipped.
+ *
+ * usagestr strings should be marked for translation with N_().
+ *
  * Returns the number of arguments left in argv[].
+ *
+ * In one-shot mode, argv0 is not a program name, argv[] is left
+ * untouched and parse_options() returns the number of options
+ * processed.
  */
 int parse_options(int argc, const char **argv, const char *prefix,
 		  const struct option *options,